The former Black Eyed Peas star and the 45-year-old actor put out all the stops for the early celebration for their little boy, whose actual birthday is on Wednesday (29.08.18), with the singer even dressing up as Wonder Woman.
She captioned a cute snap with Axl who was also in costume: ''happy bday party day to my favorite superhero #axljack (sic)''
Whilst Josh shared a clip of an ice cream truck arriving to treat the kids and took his followers on a tour through the house, which saw a large number of kids all dressed up play fighting around the pool.
He added a video on his profile with the caption: ''Thanks for showing up for my boy's birthday party @coolhaus kids went crazy. (Love you AM) #ibuywomenowned (sic)''
The reunion comes a month after the 'Transformers' star split from his actress girlfriend Eiza Gonzalez.
The pair started dating in May, following Josh's split from estranged wife Fergie last year, and decided to call time on their romance after realising they were on ''different pages'' when it came to the future of their relationship.
A source said at the time: ''Josh Duhamel and Eiza Gonzalez have ended their relationship. Josh truly enjoyed Eiza's company but is ready to move on. Eiza was very busy with work and so was Josh. She was willing to do whatever it took to make it last, but Josh just isn't in that place at the moment.
''While they did end things partially because of their work schedules, they also called it quits because they truly are on different pages and the timing just wasn't right. She was looking for a serious relationship and wanted a life partner. He wasn't at that point.
''Josh is highly independent and he values his space and free time. It was hard for Eiza to deal with being apart so much.''
The insider claimed the 'Love, Simon' actor was still mourning the split from Fergie, and ''didn't have the same goals'' as Eiza, 28, who was looking to settle down.
They added: ''Josh and Fergie's break up is still very raw and painful for him. He worked hard to move on from his eight-year marriage and Eiza was a wonderful companion for him during a very difficult time.
''After Josh and Fergie split, he wasn't expecting to get involved with anyone so soon and, in the end, Josh didn't have the same goals as Eiza. Josh and Fergie have been talking often. He has done everything to keep their parenting relationship intact, but he also misses her.''
Some friends in Josh's inner circle were said to believe the end of his romance with Eiza could leave room for another shot with the 'Fergalicious' hitmaker instead.
The source said: ''Some of their friends are relieved that the relationship with Eiza is over and hope one day Fergie and Josh will work it out.''
His new album Underneath It All is out now.
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